In recent years, along with the growth of our information society, there are increasing needs for technology capable of joint use of large amounts of information. The semiconductor field is making progress in miniaturization. Current leading edge technology is seeking to miniaturize devices to a minimum scale of 0.13 μm. Along with this progress, there is also a need for higher precision and greater miniaturization in device isolation technology, wiring technology and contact technology, etc. Structures with a higher aspect ratio (depth/aperture diameter) are also being proposed and technology to fabricate these structures is being sought. Besides fabrication technology, advances in measurement technology are also needed. In particular, development of methods for measuring precision on the nanometer scale are needed. More specifically, according to the semiconductor road map for the future, the current minimum hole diameter of 180 nanometers will shrink to 60 nanometers by the year 2010. Also, aspect ratios will increase from 7 to 12 so measurement down to these dimensions will also become increasingly difficult. Current technology uses the scanning electron microscope (SEM), to observe a cross-section of the specimen after splitting it open or machining it with a focusing ion beam (FIB) technology.
These types of methods using probe microscopes to measure surface structures with high aspect ratios include a method (U.S. Pat. No. 2,936,545) for discrete scanning of the specimen in a state where the probe is separated from the specimen, and the probe is moved to a measurement point in proximity to the specimen to measure the surface position. In this method, during scanning of the surface, the gap between the specimen and probe when making an actual physical measurement is larger than necessary and the probe moves at high speed to the next pixel; and when making surface measurements, the scanning is stopped, and the probe is moved in proximity to the specimen and measures the surface position.
Measurements of a specimen surfaces with a large aspect ratio and small aperture diameter require attaching an extremely slim probe as a tip of the cantilever. The elasticity (spring constant) of the probe is therefore poor horizontally and is nearly the same spring constant as the cantilever. The probe therefore warps or deforms on reaching the oblique surface (of the specimen).
In atomic force microscopes (AFM) of the conventional art with contact type digital probing, the probe at the pixel position, repeatedly moves close to the specimen surface and then back. When the probe captures the specimen surface while nearing it, and when the oblique surface is steep, the cantilever 80 and probe 81 are twisted as shown in FIG. 10, and an error appears in the position measurement on the surface of the specimen 82. The measurement position error Δr (surface interior) and Δz (perpendicular direction) are expressed as follows due to the warping of the probe.Δr=Fc tan θ/k(k=El/(5γa3), γa=l/t)  (1)Δz=Δr tan θ  (2)Fc=F cos2θ  (3)
Here, Fc is the fixed atomic force, θ is the angle the oblique surface tilts from perpendicular relative to the probe, k is the spring constant of the probe, E is the Young's modulus of the probe, l is the probe length, and t is the probe thickness. The γa is here called the probe aspect ratio. The probe aspect ratio holds roughly the same significance as the specimen aspect ratio. This type of measurement cannot be performed unless the probe aspect ratio is higher than the specimen aspect ratio.
In contact type methods for detecting atomic force such as the light deflection method, the force setting is approximately 10−8N. For example, when the atomic force Fc setting is 10−8N, the angle θ is 45 degrees, the Young's modulus E of the probe is 2×1011N/cm2, the probe length l is 1 μm, and the probe thickness is 50 nanometers (γa=20), then the Δr and the Δz are 2 nanometers. When the angle θ is 80 degrees, then the Δr is 11 nanometers and the Δz is 64 nanometers. Further, when the angle θ is 85 degrees, the Δr is 23 nanometers, the Δz is 261 nanometers, it can be seen that the probe tip will slip on the specimen surface. This slippage shows that the technology of the conventional art is not capable of accurately measuring the shape of surface structures having a high aspect ratio.
In view of the problems with the conventional art, the present invention has the object of providing a scanning probe microscope capable of accurately measuring surface structures with high aspect ratio, and a method for measuring surface structures of specimens having a high aspect ratio.